34 / Basics of Environmental Science
illustrates six of the commonest patterns, but others are possible and real patterns are seldom so
clearly defined as the pictures may suggest. Climate, the type of rock, and the extent of erosion all
play a part in determining the type of pattern that will develop. Dendritic patterns, for example,
usually form on gently sloping land of fairly uniform geologic character. Radial patterns occur around
domed hills and batholiths, and trellis patterns where rivers cross, more or less at right angles,
alternating bands of relatively hard and soft rocks.
As they flow, rivers can be approximately classified into zones, mainly on a biological basis. The
headstream, or highland brook, is small, usually torrential (which means its water flows at more than
90 cms-1), and the water temperature varies widely. Few aquatic animals can survive in it. A little
lower, trout can survive in what is still a fast-flowing stream, the troutbeck. Silt and mud begin to
collect at the bottom of the minnow reach or grayling zone, some plants can survive, and the animal
life becomes a little more diverse. In the lowland reach or bream zone the water flows slowly, the
river is often meandering, and animal life is diverse. In this final zone the river flows across the
coastal plain into the estuary.
10. Coasts, estuaries, sea levels
It seems natural to think of an estuary in terms of the river flowing into it, to see it as the end of the
river, with a boundary somewhere offshore where the river meets and merges with the sea. Stand on
a headland overlooking an estuary and this is how it looks, but the picture is misleading. An estuary
is more accurately described as an arm of the sea that extends inland and into which a river flows. An
estuary is dominated by the sea rather than its river, and many estuaries are in fact ‘rias’, or ‘drowned
river valleys’, old river valleys which were flooded at some time in the past when the sea level rose.
The estuaries of south-west England are good examples of rias. In several cases, such as the Camel
in north Cornwall, before the marine transgression that began about 10300 years ago the sea was 36
m below its present level (the sea is still rising at about 25 cm per century), and gently undulating
land, with hills formed from igneous intrusions through Devonian slate which survive now as offshore
islands, extended up to 5 km from the present coast. This land was blanketed with mixed deciduous
forest. Remnants of the forest have been found on the sea bed at several points along the coast and its
botanical and faunal composition determined (JOHNSON AND DAVID, 1982).
Sea levels change and at various times in the past they have been both higher and lower than they are
today, and they are changing still. During glacial periods (ice ages), sea levels fall, because the
volume of the oceans decreases as water evaporated from them accumulates in ice sheets. As the
weight of ice depresses the land beneath it sea levels rise; as the ice sheets melt they also rise; and as
land depressed by the weight of ice rises again when the ice has melted they fall. There is clear
evidence in many places that sea levels were much lower at some time in the past. Raised beaches
can be found that are several metres above the present high-tide level. These are areas of approximately
level ground, nowadays usually vegetated, containing large numbers of shells of marine organisms.
They can have been produced only by the movement of waves and tides over them, at a time when
they formed the shore; they are ancient beaches now some distance from the sea.
The sea bed at the mouth of the Camel estuary is mainly sandy, with sand bars, and there are many
sandy beaches along the adjacent coast. Sand consists primarily of quartz grains weathered and eroded
from igneous rocks inland and transported by the river. They are deposited at the mouth of the estuary,
then transported further by tides and sea currents. As they move they become mixed with varying
amounts of sea shells, most of which are crushed to tiny fragments through being battered by harder